Electro-optical sensors are known, consisting of a plurality of photo-sensitive element or pixels, able to detect light signals and to transmit them, in the form of electric signal, to a calculator which processes them and obtains images from them. The images are then transmitted to display devices able to allow a user to see the images or information deriving therefrom.
The optical sensors were previously made using CCD technology (Charged-Coupled Device), which guarantees a very satisfactory image quality in the presence of well-controlled lighting, but which does not allow to operate in an optimum manner when there is a highly differentiated light inside the same scene, that is, with an input signal having high dynamics, more than 120 dB.
Furthermore, CCDs are not very versatile from various viewpoints: they cannot easily be integrated with complex driving circuits in a single silicon support, the so-called microchip, and it is not possible to arbitrarily select a sub-window inside the matrix sensor.
To overcome some of these disadvantages of CCDs, optical sensors have been developed based on the CMOS type silicon technology (Complementary Metal Oxide Semiconductor), able to offer a good result even in conditions of very diversified lighting inside the same scene (see, for example, Seger, Graf, Landgraf—“Vision assistance in Scene with extreme Contrast”—IEEE Micro, vol. 13 page 50, February 1993).
This result can be obtained through a compression on logarithmic scale of the signal inside the photo-sensitive element. However, this conversion, obtained for example by connecting to the photo-sensitive element junction a MOS type transistor in diode configuration, as described in U.S. Pat. No. 5,608,204, suffers from the fundamental disadvantage of providing a low image definition in the event of low lighting.
High-resolution images are obtained by means of a linear reading of the photo-sensitive element; this technique, however, has the disadvantage that it does not give the possibility of obtaining good quality images in conditions of very diversified lighting inside the same scene.
To overcome this limitation, techniques are known which allow to extend the interval of visible light inside the same scene.
These techniques are very different from each other and allow to obtain information on a linear scale in conditions of low lighting, which guarantees high definition, and linear or compressed information on different scales, according to the technique used, in conditions of average-to-high luminosity.
It is also possible to distinguish between techniques that use information contained inside a single image or those which combine information arriving from several images obtained with different exposure times, in this case called multi-integration.
Among the various alternatives that exploit information contained inside the same scene, the technique is known which uses a linear output for low lighting and obtains information at the instant when there is saturation of the linear signal in order to map the average-to-high luminosity. In order to do this, a comparator is used which commutes at the instant when the linear signal reaches a threshold identified by the comparator as the saturation level. As consequence of this commutation, two reference signals are stored inside two analog memories (C1, C2). The reference signals are generated outside, in fixed and pre-determined views. From the combination of the two stored signals it is possible to obtain information for luminosity which would not be mappable by exploiting the linear signal. It should be noted that the information found from the memories C1 and C2 can be combined, but this information is totally separated from what is obtained through the linear output which, at the moment the comparator starts up, becomes insignificant.
The implementation of this known technique, using two fixed reference slopes employed in a combined manner, has the disadvantage, however, that it is necessary to realize a bulky photo-sensitive element and is therefore difficult to exploit for industrial devices, which require a high level of integration in order to achieve high-performance pixels integrated into a single silicon chip.
One purpose of the invention is to achieve a photo-sensitive element for electro-optical sensors which can be integrated into a silicon support element, or substrate, of limited size, achieving a microchip, and which is suitable to provide good-quality images at a high repetition frequency both in the case of low lighting and also in the case of an input signal characterized by high dynamics.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.